In one known dissolved oxygen meter of this type, the dissolved oxygen meter is attached to a cell, to be loaded with a sample solution, and measures partial pressure of oxygen (oxygen concentration) in the sample solution, with a sensor surface of the dissolved oxygen meter being in contact with the sample solution.
When calibrating sensitivity of said dissolved oxygen meter, as described in Patent document 1 and Patent document 2, the dissolved oxygen meter is lifted out of the sample solution to the air. The dissolved oxygen meter is then subjected to a span calibration by using the partial pressure of oxygen (oxygen concentration) in the air as a reference value.
The above calibration method is easy to conduct. However, when a partial pressure of water vapor in the air is changed due to, for example, temperature and moisture, the partial pressure of oxygen (oxygen concentration) in the air is changed accordingly. Consequently, the reference value may vary, and fail to ensure accurate calibration.
Furthermore, depending on the measurement site, it can be difficult to pull the dissolved oxygen meter out of the cell. It can therefore be difficult to use the above calibration method at some measurement sites.